IFLP   13074
Unidad Ejecutora - UE
congresos y reuniones científicas
Mössbauer Thermal Scan study of the spin crossover transition in [Fe(Htrz)2(trz)](BF4)
Vienna, Austria
Conferencia; International Conference on the Applications of the Mössbauer Effect; 2009
Spin Crossover systems [1] are very interesting materials because of the ability to change their  magnetic state, between a low-spin and a high spin configuration, under the influence of external stimuli (temperature, electromagnetic irradiation, pressure, etc.) being interesting candidates e.g. for memory devices. Among them, the [Fe(trz)3]X family (trz: triazole derivative) is a promising candidate for exhibiting bistability [2]. For the specific case of [Fe(Htrz)2(trz)](BF4), the Fe(II) ions are coordinated by the ligands forming a 1-D polymeric structure exhibiting a cooperative spin transition between the low spin (S=0) state to a high spin (S=2) state at 386 K (rising T) or 343 K (lowering T). We have prepared [Fe(Htrz)2(trz)](BF4) as bulk material and as 11 nm nanoparticles (using conventional reverse micelle techniques [3]) In order to observe a sufficiently detailed account of the spin-crossover transition, a selected Doppler energy ROI (region of interest) scan was performed. The experimental system developed in our laboratory allows the selection of ROIs in any spectral region, with a very high flexibility. The ROI can be composed of any number of points (up to 4098) with a freely selectable energy for each point. For the present case a ROI of 51 equally spaced points, approximately centred at the isomer shift of the high temperature phase was chosen; it also included one point at an energy corresponding to background transmission for control and normalization purposes. The ROI was periodically swept at 0.5 points/s while temperature was increased and then decreased at 0.23 K/min. This approach provided a detailed temperature and Doppler energy observation of the transition leading to a wealth of experimental information. Additional information on the experimental setup can be found elsewhere [4]. Figure 1 is a bidimensional Doppler velocity vs time representation of the scan performed on the bulk sample, where the transmission counts are indicated by the dots gray tone (in a colour scale in the original). The inset shows the temperature – time dependence. The plot clearly shows the transition from a low spin configuration at low temperature to a high spin one at high temperature and vice versa. The low and high spin configurations correspond to isomer shifts of 0.31 and 0.85 mm/s, and quadrupole splitings of  0.3 and 2.2 mm/s, respectively, in agreement with previous results[5]. At this low heating rate it  was observed that the transition displays hysteresis being centred at 370.7 K (heating up) and  340.5 K(cooling down) with a temperature transition region of about 5 K, in reasonable agreement with calorimetric, magnetic and X-ray absorption experiments [5] (not shown here). The data have been analysed as a whole with a temperature and energy dependent theoretical model which provides a quasi continuous description of the transition, in terms of subspectra intensities and hyperfine parameters variation. A detailed description of the used fitting function and results will be given. The nanoparticles size distribution was studied by Dynamic Light Scattering. A mean size of 11 nm was determined. The spectrum temperature evolution was followed by a Doppler energy ROI scan, similar to the one carried out for the bulk sample. Details of the spincrossover transition in the nanoparticles system are discussed in comparison to the one observed in the bulk material.[1] P. Gütlich, P. J. van Koningsbruggen, F. Renz, Struct. Bonding, 107 (2004), 27[2] J. G. Haasnoot, Coord. Chem. Rev., 200 ( 2000) 131[3] E.Coronado, J.R.Galán-Mascarós, M.Monrabal-Capilla, J.García-Martínez, and P.Pardo-Ibáñez, Adv.Mater., 19 (2007) 1359 [4] P. Mendoza Zélis, G.A. Pasquevich, A. Veiga, M.B. Fernández van Raap and F.H. Sánchez, Hyperfine Interact., in press.[5] J. Kroeber, J-P. Audiere, R. Claude, E. Codjovi, O. Kahn, J. G. Haasnoot, F. Groliere, C. Jay, A. Bousseksou, Chem.Mater. vol:6, (1994) 1404–1412